1. Field of the Invention
The present invention relates to the recovery of oil from subterranean petroleum reservoirs, and more particularly, to an improved surfactant flooding process and composition applicable to alkaline surfactant flooding (ASF) and alkaline surfactant polymer flooding (ASPF) which results in ultra-low interfacial tensions with brine against crude oil even while the surfactant present is at or below its critical micelle concentration (CMC). The alkaline surfactant flooding process is used in oil reservoirs where a primary surfactant system has been diluted with brine and pumped downhole where the alkali, which is usually sodium hydroxide or sodium carbonate, reacts with the residual oil acidic organic component(s) to form a secondary surfactant system. This "in situ" formed secondary surfactant helps the primary surfactant further reduce the interfacial tension between the residual oil and the injected fluid thereby allowing the removal of residual oil from the pores of the reservoir. The present invention utilizes a primary surfactant system which includes anionic surfactant(s), nonionic cosurfactant(s), solvent(s), and strong base.
Ultra-low Interfacial Tension Measurements (&lt;10.sup.-2 mN/m) are obtained between the diluted primary injection solution and the residual oil allowing the primary injection solution to permeate the reservoir oil thereby allowing maximum contact between the alkali and the acidic organic component(s) of the residual oil even though concentrations of the surfactants in the diluted primary system may fall to levels at or below the CMC.
2. The Prior Art
It is well known that substantial amounts of oil remain in subterranean petroleum reservoirs after primary and secondary recovery processes have been employed. Numerous tertiary means of recovering residual oil have been developed, such as adding various chemicals to an aqueous reservoir flooding medium. These processes have provided improved tertiary oil recovery in selected oil fields with suitable chemical and physical parameters.
The prior advances have not supplied an aqueous surfactant system and process which performs well at low surfactant concentrations, or surfactant concentrations at or below its CMC. Additionally, the surfactant used by the prior advances has been found to interfere with the ability of the polymer to increase the viscosity, the surfactant used is rapidly lost through absorption onto the formation, the range of surfactant concentrations where the interfacial tension of the surfactant is ultra-low (&lt;10.sup.-2 mN/m) is too narrow, the temperature range of the surfactant where the interfacial tension is ultra-low (&lt;10.sup.-2 mN/m) is too narrow, the range of alkalinity where the interfacial tension of the surfactant is ultra-low (&lt;10.sup.-2 mN/m) is too narrow, the surfactant is not readily soluble or dispersible in the formation brine, the viscosity of the concentrated surfactants is very high making it difficult to handle during transfer and dilution.
The uniqueness of the present invention is that a surfactant formulation has been found which when combined with a solution containing alkali and optionally a polymer allows the alkali to enter the pores of an oil bearing formation by lowering the interfacial tension between the oil and injection solution resulting in intimate contact between the alkali and the residual oil containing acidic organic component(s). This further allows the formation of a secondary surfactant which is formed in situ by the reaction of the injected alkali and the residual oil containing acidic organic component(s). The invention is stable in solutions of the alkali, does not degrade or interfere with the performance of the polymer, and is effective at low concentrations at or below the CMC, and works over a wide range of alkali concentrations. It is very important that the surfactant system work over a broad range of alkali concentrations because the concentration of the alkali and surfactant changes as the alkali is depleted by reactions with residual oil containing acidic organic component(s), and as the alkali and surfactant is diluted with formation brine or adsorbed onto the formation. The surfactant system must be able to function effectively during the concentration changes encountered. Accordingly the inventors have found that these problems and others have been overcome by carefully selecting the optimum mixture of surfactants, solvent(s) and co-surfactant(s), such surfactants being the various products of the sulfonation of alkylaromatics.